Stents are small and expandable tubulur members, which are inserted into clogged blood vessels to restore proper blood flow, or to treat other damaged passageways or lumens in the body, like bronchi or the esophagus. In turn, placing a stent implant (stenting) is often a catheter-based procedure. In the example of vascular stenting, a catheter is used to place a stent into a diseased artery to maintain the vessel patency after balloon angioplasty. In another application, covered stents (called stent-grafts) are also used to treat aneurysms, including abdominal aortic aneurysms. In a stent-graft procedure, the physician prevents blood from filling the aneurysm (bulge in artery) by placing a stent graft at the aneurismal site.
Generally there are two classes of stents: balloon-expanded stents, and self-expanding stents. Balloon expanded stents are initially small enough to enter the body lumen easily and are fitted over a collapsed balloon. The stent is then expanded through plastic deformation as the balloon is inflated. After inflation, the stent is in tight approximation to the lumen wall. The specific design of the stent struts (mesh segments) is optimized to provide a flexible, unexpanded structure that can track through the body cavities during insertion, and a patent lumen after expansion. Self-expanding stents are naturally sized for tight fit in the target lumen, but held in a compressed state during delivery through the body lumens to the target site. Once the self-expanding stent is located at the target lumen site, then the stent is release and allowed to spring back to its natural, expanded size. Design considerations similar to those followed with balloon-expanded stents are used in the design of a self-expanding stent. The self-expanding stent must track through the vasculature in the collapsed state and fit the lumen when expanded to provide patency. The struts of the self-expanding are designed to elastically deform during compression and return to a predetermined final shape. Those can be made of a wire-mesh or a specially designed pattern of slots or apertures.
Specific stent strut structures include wire-mesh, specially designed pattern of slots or apertures, coiled springs, helical wound spring coil, expanding forms in a zig-zag pattern, diamond shaped, rectangular shaped, and other mesh and non-mesh designs. Stents come in different sizes and designs depending on the many factors. Some of the factors include: 1) the size of the artery in consideration; 2) where the blockage is located; 3) the extent of the blockage; 4) the extent of blockage in other arteries; 5) the strength of the heart muscle; and 6) the interaction of the implanted stent with the vascular (or target lumen) physiology.
Despite the number of different considerations used in the designing a stent, the variety of materials used is quite limited. For example, stainless steel is the most common metal used for stents although, nitinol is also gaining wide-spread acceptance.
The quality of stent's function can be measured in terms of acute performance and chronic performance. Acutely, the stent keeps the lumen wall from recoiling after balloon expansion, and keeps dissected flaps from causing acute closure at the angioplasty site. Chronic performance of a stent is gauged by the degree of restenosis (re-blockage) in the treated lumen. Restenosis is considered to be a proliferative cellular response to the injury caused during angioplasty and stent implantation. Approximately 20 percent of stents close (restenose) within six months of placement.
Improving stent performance can be measured against several other criteria: 1) designing smaller diameter stents (less than 2.5 millimeters) for smaller vessels; 2) custom-designing stents for an optimal fit; 3) designing stents for multiple sites within the same artery (including stents with side branches); and 4) providing effective coating of stents with anticoagulants and antiproliferation agents.
Unfortunately, current materials used in stents are not readily adaptable to many of these desired improvements. The limitations of the current stent materials include both limited fabricability, and non-optimal physical and mechanical properties. For example, the mechanical properties of stainless steel and nitinol depend on the history of thermo-mechanical process history. As such, various fabrication and finishing steps can result in inconsistent or inferior physical and mechanical properties. Furthermore, the physical and mechanical properties of current materials are not generally sufficient for the development of new novel stent designs, such as stents having diameters less than 2.5 mm.
Accordingly, a need exists for a new class of materials to address the material and fabrication deficiencies of current materials as well as to provide options and tailorable properties for the various demands of stents.